This invention relates to the stabilisation of aqueous hydrogen peroxide solutions, especially under alkaline conditions. The invention also relates to the preparation of storage-stable alkaline hydrogen peroxide solutions which can be used as a base for liquid bleach products.
Hydrogen peroxide decomposes to water and oxygen and cannot be kept for long unless it is properly stabilised.
Decomposition of hydrogen peroxide occurs mainly in a catalysed reaction, of which the active catalysts are trace amounts of indigenous transition metal ions, such as copper, iron, cobalt and manganese ions, and as such can in principle be reduced by adding suitable metal complexing agents. Indeed many stabilisers for hydrogen peroxide known in the art are complexing agents and just as many stabilisers have been suggested and used for solving, at least partially, the decomposition problem of hydrogen peroxide solutions.
While the stabilisation of hydrogen peroxide is relatively easy in acid solution, i.e. at pH&lt;&lt;7, where the concentration of reactive HO.sub.2.sup.- ions--the active form of hydrogen peroxide--is minimal, it has heretofore been considered as practically impossible to achieve a satisfactory stabilisation of hydrogen peroxide under alkaline conditions. The term "alkaline" as used here is meant to indicate alkaline pH values of above 7, particularly between 8 and 14.
Indeed many complexing agents developed heretofore as stabilisers for use with hydrogen peroxide solutions and which give satisfactory stabilisation of hydrogen peroxide under acid conditions, are poor stabilisers and hence unsuitable when used with hydrogen peroxide under alkaline conditions, particularly at pH above 8.
For this reason hydrogen peroxide is presented commercially as product concentrates having a pH lying generally within the range of 2-5, mostly between 2 and 3. Acid hydrogen peroxide is, however, unsuitable for use as a bleach due to the low concentration of HO.sub.2.sup.- ions, the active form of hydrogen peroxide. For effective bleaching an alkaline pH is a necessary requirement, the higher the pH the higher the HO.sub.2.sup.- ion-concentration to provide for more effective bleaching. The selection of pH used will generally depend upon the substrate which is to be bleached. For textile bleaching a suitable pH is within the range of 8-12.5, preferably between 9 and 11.5, as higher pH's will tend to cause excessive fabric damage.
When said commercial acid hydrogen peroxide solution is made alkaline, e.g. with sodium hydroxide, to a suitable bleaching pH with or without dilution with water to any desired concentration, it becomes unstable and cannot be stored without quickly losing its activity.
Stabilisers which have been suggested for use with hydrogen peroxide solutions include dipicolinic acid (DPA) as described in U.S. Pat. No. 2,624,655; ethylenediamine tetra acetic acid compounds (EDTA) as described in British Pat. No. 1,285,151; amino tri-(lower alkylidene phosphonic acid) compounds, e.g. sodium nitrilo tri-(methylene phosphonate) as described in British Pat. No. 1,119,221; alkylidene-diphosphonic acid derivatives, e.g. ethane-1-hydroxy-1,1-diphsophonate (EHDP) as described in British Pat. No. 925,373; and ethylenediamine tetra methylene phosphonic acid) as described in U.S. Pat. No. 3,701,825.
Most of these known stabilisers, though having some ability to inhibit decomposition of hydrogen peroxide under acid conditions, are however poor stabilisers for alkaline hydrogen peroxide solutions, and are suitable or even unsuitable for producing satisfactory storage-stable alkaline hydrogen peroxide solutions.